Fischer-Tropsch synthesis is an effective process for converting synthesis gas containing hydrogen and carbon monoxide, also referred to as syngas, to liquid hydrocarbon fuels. It is well known that Fischer-Tropsch synthesis involves a polymerization reaction beginning with a methylene intermediate to produce a wide distribution of hydrocarbons ranging from light gases to solid wax. Hybrid Fischer-Tropsch catalysts, also referred to interchangeably as “hybrid FT catalysts” or “HFT catalysts,” have been developed containing both a Fischer-Tropsch synthesis component, e.g. cobalt, and an acidic zeolite component which have been found to be capable of limiting chain growth in the polymerization reaction to provide a more desirable product distribution.
Challenges have been encountered in hybrid Fischer-Tropsch catalysts containing cobalt as a result of the strong interaction between the cobalt and the zeolite. These may include lower than desired catalytic activity, lower than desired degree of cobalt reduction and undesirably high methane selectivity. For example, the activity of some hybrid Fischer-Tropsch synthesis catalysts which have been reported is about 0.2 g of C5+/gcat/h (U.S. Pat. Nos. 7,973,087; 7,973,086; 7,943,674; and 7,825,164). Generally, it is preferred that the activity of a catalyst be higher.
Another challenge in the development of improved hybrid Fischer-Tropsch catalysts is the development of catalysts which are active, stable and provide high C5+ productivity. Deactivation of hybrid FT catalysts can occur due to a variety of causes, including sintering, surface carbon formation, cobalt-support mixed compound formation, cobalt oxidation and poisoning. Deactivation of HFT catalyst results in reduced yields of desired products. Catalyst deactivation by some of these can be addressed by regeneration. However, some modes of deactivation lead to permanent deactivation from which the catalyst cannot be regenerated. Additionally, high water partial pressures can oxidize active cobalt metal to inactive cobalt oxide. Water partial pressure above a certain value leads to high rates of catalyst deactivation. This puts a limit on the maximum per-pass carbon monoxide conversion that a catalyst can experience for acceptable deactivation. Accordingly, it is important to improve water resistance from the point of view of catalyst activity and catalyst life. Common poisons for FT catalysts include sulfur, nitrogen-containing compounds such as hydrogen cyanide and ammonia. Catalyst deactivation by sulfur cannot be regenerated. Other modes of deactivation include coalescence of cobalt metal crystallites leading to a loss of metal surface area. Catalyst deactivation by this mechanism cannot be regenerated.
Hybrid Fischer-Tropsch synthesis also generally produces a large percentage of olefinic hydrocarbons. An olefinic hydrocarbon is defined as a hydrocarbon in which one or more double bonds exist within the molecule. Olefinic, or unsaturated, hydrocarbons have the potential to be disruptive to refining processes, creating problems including crude heater and preheat train fouling, storage instability and gum deposits. Furthermore, the hydrogenation of olefins, apart from diene saturation, is not practiced in crude oil refining. For this reason, synthetic hydrocarbon mixtures must be treated so as to substantially remove unsaturated hydrocarbons before being blended into crude oil.
It would be desirable to have a means for converting synthesis gas to a hydrocarbon mixture having a low percentage of olefins. There remains a need for hybrid Fischer-Tropsch catalysts with improved catalytic activity which provides improved productivity in a desired range of product distribution, i.e., C5+.